Relationship between T_reg frequencies and prospective risk of malaria.

<p>¹ –Multivariate is adjusted for prior malaria incidence</p><p>Relationship between T_reg frequencies and prospective risk of malaria.</p

Decline of FoxP3+ Regulatory CD4 T Cells in Peripheral Blood of Children Heavily Exposed to Malaria

<div><p>FoxP3+ regulatory CD4 T cells (T_regs) help to maintain the delicate balance between pathogen-specific immunity and immune-mediated pathology. Prior studies suggest that T_regs are induced by <i>P</i>. <i>falciparum</i> both <i>in vivo</i> and <i>in vitro</i>; however, the factors influencing T_reg homeostasis during acute and chronic infections, and their role in malaria immunopathogenesis, remain unclear. We assessed the frequency and phenotype of T_regs in well-characterized cohorts of children residing in a region of high malaria endemicity in Uganda. We found that both the frequency and absolute numbers of FoxP3+ T_regs in peripheral blood declined markedly with increasing prior malaria incidence. Longitudinal measurements confirmed that this decline occurred only among highly malaria-exposed children. The decline of T_regs from peripheral blood was accompanied by reduced <i>in vitro</i> induction of T_regs by parasite antigen and decreased expression of TNFR2 on T_regs among children who had intense prior exposure to malaria. While T_reg frequencies were not associated with protection from malaria, there was a trend toward reduced risk of symptomatic malaria once infected with <i>P</i>. <i>falciparum</i> among children with lower T_reg frequencies. These data demonstrate that chronic malaria exposure results in altered T_reg homeostasis, which may impact the development of antimalarial immunity in naturally exposed populations.</p></div

Regulatory T cells decrease over time in individuals with high but not low malaria incidence.

<p>T_reg frequencies of CD4+ T cells (FoxP3+CD25+CD127^dim) were measured at 16, 24 and 36 months of age (PROMOTE- SP arm). <b>(A)</b> Children were divided into tertiles based on incidence of malaria between 16 and 36 months; lowest tertile median incidence 4.0 episodes ppy (IQR 3.0–5.5); intermediate tertile median incidence 8.2 episodes ppy (IQR 7.3–9.2); highest tertile median incidence 13.7 episodes ppy (IQR 10.3–14.6). The median duration since last malaria infection in these three tertiles was 8.5, 20, and 100 days, respectively. Wilcoxon matched pairs signed rank test p values indicated. Between 16 and 36 months, T_reg frequencies declined in individuals in the highest but not lowest tertile of malaria incidence. <b>(B)</b> Changes in T_reg frequencies between 16, 24 and 36 months were compared between children in the lowest, intermediate, and highest tertiles of malaria exposure by generalized estimate equations, accounting for repeated measures, age, duration since last malaria episode and parasite status at time of sampling.</p

TNFR2 expression on FoxP3+ regulatory T cells declines with increasing prior malaria incidence.

<p><b>A.</b> Frequencies of TNFR2 expressing T_regs (FoxP3+CD25+CD127^dim) were compared between children in the high malaria transmission area (Tororo District) and in the low transmission area (Jinja district). TNFR2 expression on T_regs was higher in low compared to high transmission settings. Wilcoxon signed rank test indicated. <b>B.</b> The association between frequencies of TNFR2 expressing T_regs and number of recent malaria episodes was analyzed among children from high malaria exposure area (Tororo District). TNFR2 expression declined with increasing number of malaria episodes in last 90 days. Regression coefficient and p value are indicated.</p

Regulatory T cells decline with increasing prior malaria incidence and mosquito exposure among children in a high transmission setting.

<p>Regulatory T cell frequencies were analyzed as the percentage of FoxP3+CD25+ of CD4+ T cells from <b>(A)</b> fresh whole blood in 2-year old (PROMOTE-cohort, no chemoprevention control arm) and <b>(B)</b> and frozen PBMCs from 4-year olds (TCC) and the association with prior malaria incidence analyzed. In both 2 and 4 year olds, T_reg frequencies declined with increasing prior malaria incidence. <b>(C/D)</b> Regulatory T cell frequencies, analyzed as the percentage of FoxP3+CD25+CD127^dim of CD4+ T cells from 1 to 11 year old children (PRISM cohort, high transmission Nagongera, Tororo District), declined with increasing mean daily household mosquito exposure (from monthly CDC light traps) <b>(C)</b> and age <b>(D)</b>. <b>(E)</b> The relationship between T_reg frequencies and age was analyzed in children from the low transmission Jinja District; there was no decline in T_reg frequencies with age in children from the low malaria transmission settings. For all analyses, Spearman’s rho and p are indicated.</p

FoxP3+ regulatory T cells are increased in high compared to low transmission settings, but decrease with age only in highly exposed children.

<p><b>(A)</b> FoxP3+CD25+CD127^dim regulatory T cell frequencies in children age 1 to 11 years (PRISM cohort) residing in the low transmission Jinja District (n = 34) were compared to children in the high transmission Tororo District (n = 91). Wilcoxon ranksum p value indicated. <b>(B</b>) FoxP3+CD25+CD127^dim regulatory T cell frequencies were compared between children from low and high transmission areas at age 1–4 (n = 11 and n = 18), 4–7 (n = 14 and n = 35) and 7–11 (n = 9 and n = 38) years of age and adults (n = 9 and n = 37). Wilcoxon ranksum for age group comparisons, p values indicated.</p